1. Field of the Invention
The present invention generally relates to a power conversion system, and more specifically to a power conversion system dynamically adjusting the switching frequency of the pulse width modulation driving controller based on input voltage and loading status.
2. The Prior Arts
With remarkable advance in the electronic technologies, various electronic devices have been successfully promoted to the market. Since the electric appliances need sufficient electric power to operate, the manufacturers have constantly developed various power converters to meet the different requirements. For example, 110V AC (alternating current) city power is converted into 12V DC power to drive the DC motor, or 5V DC (direct current) power for providing the electronic devices, even 3.3V, 2.5V or 1.8V for the ICs (integrated circuits). Besides, it is sometimes needed to convert lower voltage power into higher voltage power, like the inverter. For instance, 12V DC power from the battery is converted into 110V or higher AC power.
One of common power converters is the switching power converter, which can be implemented in the buck converter, boost converter, buck-boost converter, forward converter, flyback converter, half bridge converter or full bridge converter. Especially, the flyback converter has the advantages of simple architecture, low cost and wide range of voltage regulation, and thus is widely used in the electronic devices with moderate or low power consumption.
The traditional flyback converter comprises Resonant, QR (Quasi-Resonant), Active Clamp types, and primarily employs aero voltage and/or zero current switching to achieve the aim of reducing switching loss due to switching elements (generally power transistors), thereby increasing the efficiency of power conversion.
For the above QR technology, the flyback converter operates in DCM (Discontinuous Conduction Mode) to effectively reduce the inductance so as to achieve aero voltage and/or zero current switching. Additionally, the QR technology may utilize the scheme of Valley Switching to improve the efficiency, and the variant frequency to suppress the EMI issue.
However, one of the primary shortcomings in the prior arts is that the QR flyback converter generally operates in DCM such that the peak current and the RMS (root-mean-square) current are considerable high, especially in heavy loading. As a result, the switching elements and the output diodes may generate higher conduction loss and turn-off switching loss, and the internal of the electric transformer generates higher ferric core loss, such that the efficiency of power conversion is greatly reduced. Therefore, it is not suitable for higher output power, such as more than 150W.
Additionally, as for EMI and power conversion efficiency, the industrial standard for the power-consuming electric appliances in the future is sure to be stricter. Therefore, it is greatly needed for a power conversion system with adjustable frequency, which reduces the EMI issue resulting from the switching appliance in the buck mode, removes the output jitter/ripple and greatly increases the efficiency of power conversion, thereby overcoming the above problems in the prior arts.